CI: Add Drone pipeline to check that Alerting Swagger API spec is up to date

[rwwiv]

What is this feature?

This PR introduces a new Drone pipeline that will verify the Swagger spec for Alerting is up to date with the definitions and fail CI if this isn't the case. This should be easy to extend to cover all Swagger definitions later as well.

Why do we need this feature?

Currently a developer needs to be sure to run the relevant make command whenever they make changes to a Swagger definition in Alerting. This can lead to the spec drifting from the definitions.

Who is this feature for?

Users of the Alerting Swagger docs.

Which issue(s) does this PR fix?:

/

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[rwwiv]

This is blocked by go-swagger/go-swagger#2783, closing for now.

Several paths forward exist:

1. Fix the bug above.
2. Use adapters between our models and external structs.